1. Field of the Invention
This invention relates to fibrous substrates useful in the manufacture of carbon fiber/carbon matrix composites, and to carbon fiber/carbon matrix composites manufactured therefrom. Representative of such composites are aircraft and high performance automotive brake discs made by depositing carbon matrices on carbon fiber substrates of this invention and subsequently carbonizing the combinations to provide carbon matrices that are reinforced with carbon fibers.
2. Related Art
Many advances have been made over the years in the art relating to brake discs.
U.S. Pat. No. 5,388,320 describes the manufacture of carbonizable needle-punched filamentary structures (typically, annular performs) made up of layers of unidirectional filaments and staple fibers. These structures can be used to make shaped articles (typically, brake discs) of carbon reinforced with carbon fibers. As taught in column 7 of the patent, some of the arc segments used to make up the structures are cut in such a way that the majority of the filaments extend substantially radially of the eventual annulus, while others are cut so that the majority of the filaments extend substantially chordally of the annulus. The former segments have greater dimensional stability in the radial direction and the latter segments have greater dimensional stability in the chordal direction.
U.S. Pat. No. 5,546,880 describes fibrous substrates for the product of carbon fiber reinforced composites comprising multilayered annular shaped fibrous structures, suitable for use in the manufacture of friction discs, made from multidirectional fabric, that is, fabric having filaments or fibers extending in at least two directions.
The present invention involves the recognition that, in carbon fiber composite friction linings, the orientation of the fiber at the friction surface plays a major role in the wear characteristics of the material. When fibers of opposing direction on the friction surfaces slide against each other, mechanical wear takes place and the fiber bundles are torn form the friction surface. This fiber pull-out leads to breakdown of the surrounding matrix of carbon. As more areas of fiber pull-out occur on the friction surface, the matrix surrounding these fibers also breaks down to fill the voids created. This results in a reduction in the overall thickness of the frictional material.
This invention addresses the need of both brake manufacturers and their customers, by increasing the field life (via reduction of the wear rate) of carbon fiber friction materials and thereby reducing the cost of ownership.
Methods for manufacturing annular preforms made from tows of oxidized polyacrylonitrile continuous filaments are described in U.S. Pat. No. 5,388,320, the entire contents of which are hereby expressly incorporated by reference. In the new preform technology of the present invention, fiber orientation in the preform is in the radial direction. This means that the continuous fibers run mainly from the inner diameter to the outer diameter of the annular disc. By orienting the fibers in this fashion, fiber pull-out is minimized, thereby reducing mechanical wear. Testing has shown that by using this preform fiber architecture, wear rates can be reduced up to 40 percent while maintaining disc strength and integrity.
One embodiment of this invention is a carbon fiber brake preform comprising an annular disc built up of fabric arc segments composed of from 90 to 70 weight-% continuous fibers and from 10 to 30 weight-% staple fibers. A typical annular disc of this invention may, for instance, be composed of 85 weight-% continuous fibers and 15 weight-% staple fibers. Preferably, both the continuous fibers and the staple fiber are oxidized polyacrylonitrile fibers. In this preform, at least 80% of the continuous fibers in the fabric segments are arranged to be located within 60xc2x0 of radially from the inner diameter to the outer diameter of the annular disc. Thus, for instance, the fabric arc segments may be arranged with substantially all of their continuous fibers oriented in the radial direction and parallel to the segment arc bisector, or the fabric arc segments may be arranged in alternating layers in which, respectively, approximately half of their continuous fibers are oriented at a +45 degree angle with respect to the segment arc bisector and approximately half of their continuous fibers are oriented at a xe2x88x9245 degree angle with respect to the segment arc bisector.
Another embodiment of this invention is a method for making a preform composite. The method includes the steps of: a.) providing a needle-punched nonwoven fabric comprising a major portion of unidirectional continuous fiber and a minor portion of staple fiber; b.) making from this fabric a plurality of segments having the outside diameter and the inside diameter of the preform to be manufactured from the fabric; c.) arranging the segments in a multilayered intermediate to a weight and dimension calculated to provide a desired preform density for the application; d.) heating the multilayered intermediate to a temperature above 1500xc2x0 C. in an inert atmosphere for an amount of time sufficient to convert the fibers to carbon; and e.) densifying the carbonized product by carbon deposition to the desired preform density. The segments may be arranged in step c.) with their continuous fibers oriented in the radial direction and parallel to the segment arc bisector or in alternating layers in which their continuous fibers are oriented alternatively at a +45 degree angle with respect to the segment arc bisector and at a xe2x88x9245 degree angle with respect to the segment arc bisector. The carbonized product may be densified in step e.) using Chemical Vapor Infiltration/Chemical Vapor Deposition. A typical density for a finished disc produced by this method is in the range 1.70-1.80 g/cc.
Still another embodiment of this invention is a method of reducing wear in an annular brake disc which comprises manufacturing said disc from preforms reinforced with a plurality of continuous fibers in which at least about 80% of the continuous fibers are aligned in a generally radial manner, for instance within 60xc2x0 of the radii of the annular brake disc. In two specific cases, the continuous fibers are located on the radii of the annular brake disc or the continuous fibers are located at angles of 45xc2x0 from the radii of said annular brake disc. Using this method, wear of the brake disc may be reduced, for example, by 25% or more compared to wear of an otherwise comparable brake disc made from preforms in which half of the continuous fibers are located outside of the 120xc2x0 arcs bisected by the radii of each of the preform segments.
Finally, this invention provides a shaped fibrous fabric structure having an annular disc configuration and being formed of multiple, successively-stacked layers of abutting fabric arc segments composed of from 90 to 70 weight-% continuous fibers and from 10 to 30 weight-% staple fibers, the fabric arc segment layers being interconnected by at least a portion of the staple fibers, wherein at least 80% of the continuous fibers in the fabric arc segments are located within 60xc2x0 of radially from the inner diameter to the outer diameter of the annular disc. The fabric arc segments may be arranged with their continuous fibers oriented in the radial direction and parallel to the segment arc bisector, or they may be arranged in alternating layers in which their continuous fibers are oriented alternatively at a +45 degree angle with respect to the segment arc bisector and at a xe2x88x9245 degree angle with respect to the segment arc bisector.
Implementation of these new fiber preform architectures (radial and +/xe2x88x9245xc2x0) enables the brake manufacturer to produce fewer friction linings to meet existing airline requirements. In addition, the brake manufacturer will be able to meet increasing demand without further capital investment by utilizing the excess production capacity created by this technology.
Additional advantages of the present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.